| Traditional direct coal combustion technology has been adopted in moat medium and small-sized coal fired industrial furnaces in China, which not only leads to low energy efficiency but also results in environmental pollution. In recent years, gasification technology has been adopted by some factories for environmental protection. Fixed bed gasifier account for almost 90% of the total gasifiers. The gas produced by conventional fixed bed gasifier has low heating value and high tar concentration. The kind of gas doesn’t suit direct combustion. If the hot gas is cleaned outside the gasifier and turn into cold gas without tar, the cost of equipments and running will increase and the secondary pollution will be produced. So it doesn’t suit the large numbers of medium and small-sized coal fired industrial furnaces. In order to solve the above problems, The integrated system for coal high temperature air gasification and high temperature air combustion and the coal tar-free gasifier was developed in the thesise. The high temperature air preheated by waste heat of high temperature flue gas is used for coal gasification as gasification agent in the integrated system. The tar crack and turn into fuel gas inside the gasifier during coal gasification and the clean combustion is realized. The study mentioned above not only is the theoretical foundation for the structural design of gasifier and the optimization of operational parameters, but also brings forward a new way for the energy saving and emission reduction of medium and small-sized coal fired industrial furnaces in China.1. The high temperature air as gasifying agent gasification of coal was applied successfully the first in a fixed bed gasifier by self-designed. The effects of operational parameters on gasification performance were studied. The experimental results show that the gas heating value increase 32.5% with the air pemperature increasing from 500℃to 800℃. The effects of ER and the steam/carbon ratio on coal gasification performance are realized by changing the gasification temperature, and the two parameters have an optimum operating range for a certain gasification process. It’s of significance to optimize the process parameters such as ER and the steam/carbon ratio for high temperature air gasification process. It is avail for gasification performance to adopt higher ER, lower steam/carbon ratio and higer air temperature in the condition of no slag bonding in gasifier. The different air temperature carries with them corresponding ER. The higher the air temperature is, the lower ER.2. The integrated system for coal high temperature air gasification and high temperature air combustion was developed through combining the high temperature air gasification with high temperature air combustion technology. The cold state experimental results indicate that the flow rate of gasificaiton air is increased with an increase in the opening degree of fan and a decrease in the opening degree of a fume exhaust fan. The flow rate of combustion air is increased with an increase in the opening degree of fume exhaust fan and a decrease in the opening degree of fan. The regulation range of the gasification air and combustion air are between 0～68.5Nm3/h and 0～122.5Nm3/h, which may satisfy the gasification and combustion’s needs, respectively. The hot state experimental results indicated that Integrated system made the most of the sensible heat of the gas-fired furnace’s flue gas. The gasification efficiency and thermal efficiency of the high temperature air gasification is 7.3% and 6% higher than the normal temperature air gasification. The thermal efficiency of the Integrated system is 29.14% higher than the normal hot gas-fired furnaces. It adequately shows the advantage of the Integrated system.3. Simultaneous TG-MS was used to investigate thermo-gravimetric behavior of tar and the the releasing characteristics of gaseous products during the pyrolysis. We expatiate the mechanism of tar thermal cracking. The results show that:(1) The pyrolysis behavior of tar is slow at 100℃and become quicker after 200℃, and is the quickest at 323℃, then, it slow down. When the temperature is higher than 800℃, it slowly quicken and appeares three apices at 877℃,1084℃,and 1235℃. The pyrolysis behavior of tar is over at 1300℃and the solid residue is about 49.8%.(2) The release of H2O has three stage during pyrolysis of tar, and is from adsorbed water emersion, pyrolysis water produced by condensation polymerization of hydroxy groups and the chemical bond of functional group broken, respectively. It mostly release at 316℃. H2 release between 400～1050℃obviously, then it slow down. The release rules of CH4, C2H4, and C2H2 are similar, which all have an apice which is similar to Gaussian distribution. The difference of three apices’ intensity show that the hydrocarbon release by tar pyrolysis primarily are CH4, C2H4. C3, CO and CO2 all release above 800℃obviously.4. In order to investigate the influencing factors of tar-free gasification, the high temperature air gasifier was mended. Using the mended gasifier, a series of tar-free gasification experiments were carried out in different typical operating conditions. Based on this experimental investigation, the throated twin-fire fixed bed gasifier structure was put forward and a high temperature air tar-free gasifier was developed. The results show that:(1) The injection of secondary air is an effective way to reduce the primary tar concentration in gas. The method which solely inject secondary air at pyrolysis region of gasifier can merely offer a high temperature condition for tar pyrolysis, but it is at a cost of combustion of a part of gas for updraft gasifier. It can’t get the double effect of the reduction of tar and the increase of heating value of gas. The injection of the secondary air raises the highest temperature in upper oxidation layer and the temperature in reduction layer for the throated twin-fire structure of gasifier. This can enhance reaction rate of tar pyrolysis, Boudouard reaction and water-gas reaction, and improve the concentration of CO and H2, heating value of gas and gasification effeciency. Consequently, the gasification is ameliorated.(2) The throated twin-fire gasifier and the three-stage air-feeding measure can reduce the tar concentration and raise the heating value of gas. The tar concentration and the heating value of gas can get 10mg/Nm3 and 6466.9kJ/Nm3, respectively.(3) The injection of the grate air leads to a comparable high carbon conversion ratio and also to an ash with a low carbon content. The more than ever grate air is adverse to reduce tar concentration. This kinds of gas can satisfy all kinds of burners and heating process. The tar-free gasification is realized.(4) An amount of steam avail to reduce tar concentration, but more than ever ateam will decrease the gasification temperature and increase the tar concentration in gas. |
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